Environmental Engineering Reference
In-Depth Information
consequence of a species adopting a strategy to deal with competition from
other species and predation. At the other extreme, a species could have a
small geographic range but have large local population sizes within that
range. For example, a species of primrose (
Primula scotia
) occupies a tiny part
of northeastern Scotland, but it has very large population sizes in several lo-
cations within that range (Rabinowitz et al. 1986).This pattern suggests that
the species has substantial ability to thrive in a variety of conditions of soil
moisture and fertility. Rarity due to this mechanism implies that there likely
are some areas that are suitable to this species, but it has yet to colonize
those.
The implication of this contingency is that efforts aimed at conserving
rare species can be graded in their priority by using the three features of
species populations as a decision tool (Rabinowitz et al. 1986). So, the most
critical form of rarity—a species with small range, narrow habitat specificity,
and small population size—is a species most deserving of immediate con-
servation attention because it is most likely to be jeopardized by distur-
bances that lead to loss of habitat (Rabinowitz et al. 1986). Species with
other contingent combinations grading from narrow to wide geographic
range, narrow to broad habitat specificity, and small to large population sizes
become less of an immediate conservation concern.
Habitat Fragmentation and the
Species-Area Relationship
Ever since Alexander von Humboldt's accounts of his expedition to the
Amazon basin between 1799 and 1800 (Helferich 2004), we have revered
the extraordinarily rich diversity of plant and animal life found in tropical
ecosystems. It is no small wonder, then, that any human activity that imper-
ils this diversity is likely to spark considerable attention.
In particular, vast expanses of humid tropical forests, which may harbor
half of all species on the globe, are being lost or fragmented into very small
areas as a consequence of logging and the use of fire for land conversion
into agriculture (Ferraz et al. 2003).The species-area relationship predicts
that habitat fragmentation qualitatively should lead to loss of valued tropi-
cal biodiversity, once fragments become small (figure 6.1b). So there is good
reason for concern about the fate of tropical diversity in the face of this
large-scale disturbance. But, the critical uncertainty for conservation is ex-
actly what size and extent of habitat fragmentation is permissible without
causing significant loss of species diversity.
